The present invention relates generally to airbag inflators used to inflate vehicle airbags, and specifically, to an autoignition composition which provides a means for ignition of the gas generant when an inflator is exposed to elevated temperatures.
Airbags used in supplemental occupant restraint systems in automobiles require a rapid generation of gas in order to inflate the airbag during a crash. There are two methods currently in use to supply gas for airbag inflation: a compressed stored gas and a combustible pyrotechnic material. This invention relates to the latter, combustible pyrotechnic material. The use of combustible pyrotechnic material involves housing a combustible material in a combustion chamber which has a throttling means to control the combustion pressure and thereby the rate of gas generation. The rate of gas generation for a given gas generant can also be controlled by the amount of initial surface and rate of change of the surface area, as propellant burning takes place perpendicular to the surface. The rate of gas generation determines the rate of inflation of the airbag and the type of protection afforded to the occupant during an automobile crash.
The gas produced by the burning of the gas generant must be non-toxic and meet stringent requirements. Typically, nitrogen is the desired product gas from the combustion process as it is non toxic, of low reactivity, and has a relatively low heat capacity. Nonazide gas generants are currently the preferred type of gas generant. Nonazide gas generants are preferred because they are non-toxic or xe2x80x9cgreen.xe2x80x9d Nonazide generants typically contain organic or organometallic fuels as oppose to sodium azide, which has been used in the past. The preferred fuels have low amounts of carbon and hydrogen while having higher amounts of nitrogen. Organic/organometallic fuels typically have low melting points. When formulated into gas generant containing certain oxidizer system, organic/organometalic fuels have a problem as they melt or form eutectics at relatively low temperatures. The aforementioned problem becomes a serious issue when these gas generants are subjected to high temperature aging or bonfires.
Airbag inflators are designed to have a minimum weight and operate at relatively high pressures. Lightweight airbag inflators may be made of different types of material ranging from aluminum to stainless steel. Airbag inflators and the gas generant house are designed to function at generally less than 95xc2x0 C. Melting or distortion of organic/organometallic based gas generant can occur at high temperatures resulting in a perturbation of surface area. Perturbation of the surface area of a gas generant can result in uncontrolled or undefined burning and high pressure in the airbag inflator. In order to insure that an airbag inflator functions in a safe manner at temperatures greater than the normal operating temperature an autoignition material is required.
The terms, xe2x80x9cautoignition element,xe2x80x9d autoignition composition,xe2x80x9d or autoignition materialxe2x80x9d mean a material which will spontaneously ignite or combust at a temperature lower than that which would lead to catastrophic failure (i.e. explosion, fragmentation, or rupture) of the airbag inflator upon ignition. Autoignition insures that the airbag inflator function in a safe manner and minimizes risk from deployment at temperatures outside the design limits. Elevated temperatures may be encountered in bonfires and the like. The United States Department of Transportation requires that airbag inflators function in a normal manner in a bonfire in order to obtain a shipping classification. An autoignition element is a material which ignites the gas generant in a means which result in a non-failure of the unit. The ignition takes place between the upper limits set by the end user and the melting, decomposition, or autoignition of the gas generant. An autoignition element may be a single material or a mixture, granular or compressed, formulated to autoignite at a given temperature. The autoignition element must be stable at the upper functioning limit temperature, not decompose or ignite during aging, and still function as the required temperature.
To overcome the potentially catastrophic situation of housing failure, autoignition materials are used which spontaneously combust or ignite at a temperature lower than that which would lead to the failure of the inflator housing.
U.S. Pat. No. 5,959,242 and U.S. Pat. No. 6,101,947 teach the use of metal fuels with various oxidizers in a low temperature autoignition composition.
U.S. Pat. No. 5,866,842 teaches a low temperature autoigniting composition comprising a low temperature melting oxidizer and a fuel, wherein the low temperature autoignition composition autoignites in the temperature range of about 130xc2x0 C. to 175xc2x0 C.
Basic requirements of an autoignition composition for a airbag inflator used in a vehicle occupant restraint system are that the autoignition composition be thermally stable up to 107xc2x0 C. and posses physical integrity to withstand abrasion and environmental changes. The autoignition compositions of the present invention ignite in the temperature range of 120xc2x0 C. to 160xc2x0 C. The preferred composition for the autoignition composition comprises equal weight percentages of NQ, Sb2S3, and AgNO3.